[ckelloug]

John,

Thinking about it,

As a Brit, you can probably duplicate the work of BW Staynes because you can get peculiarly British materials. Staynes used 10-12 mm Diabase Hornfels from Penlee. It looks like a googling shows that the Penlee quary has closed however. He also used No. 50-100 silica sand, pulverized fuel ash (fly ash here in the states) and Huntsman GY250 epoxy with HY830 and HY850 hardeners which are still in the Huntsman catalog and formulary.

BW Staynes got his PhD at Brighton Polytecnic. I have no idea if this is close to you as I am embarassed to admit my geography isn't good for the UK and I don't know where you are specifically, but I bet you have a much better chance of getting a hold of his thesis than us Americans do.

The info on the thesis in the paper I was reading is: STAYNES, B.W. Epoxide resin concrete as a structural material with special reference to the limit state design philosophy. Ph.D. thesis- National Council of Academic Awards, May 1972.

The only copy I can find of this thesis using the worldcat online search tool is in the University of Hong Kong. . . so It's probably the type of thing that's not exactly popular.

I am with you on the may have missed something part on the original Gamski and Gupta comment about the Specific Surface Area Based epoxy estimation. I cried BS myself after bruno first posted the paper from Gupta but after reading more, I think they may have something. Neither Gupta who cited it and led me to the idea nor Gamski who postulated it gave didly squat worth of useful explanation of the concept in either of their papers other than the equation which is shall we say a bit tricky to interpret. . .

Cheers and thanks for reading through my torturous post,

Cameron

[martinw]

I'd personally recommend looking for one with 29.75 inches and as many CFM as you can afford.
Cameron

Dear Cameron,

IMVVHO, CFM does not matter if you have a well sealed vacuum system of the types suggested for EG. I do not think it matters if you take two minutes, or indeed, two hours to get to the the level of vacuum needed to "de-air" the glop, as long as the glop is not setting up in that period.

If there are worries about the epoxy out-gassing its chemical components, they will not appear until that critical level of vacuum is reached, and you can control that with a vacuum switch controlling the pump at any level you wish. Time, and vac pump though-put do not matter,unless the epoxy is going off.


Post#1478 and #1479. Aluminium preparation.

I could be wrong, but I'm reasonably sure that most epoxy manufacturers suggest mechanical abrasion ( it increases the surface area) and solvent de-greasing to improve the bond.

Best wishes

Martin

[greybeard]

Cameron - If we start with the 30um average layer as being the way to achieve maximum strength for a given mix, it occurred to me to wonder what size single particle would achieve this in close packing. My calculation suggests a diameter of 160um. I took 74.4% of the space taken up by the particles, the remainder epoxy. This, spread round the particles gives an average layer of 15um round each one, the two layers giving the required thickness.
If you could confirm my calculation, would this suggest that the presence of particles smaller than this, inserting themselves in place of the epoxy, is going to reduce the average layer thickness, and hence the strength for this consideration only ?

I can see that the smaller particle would add strength from a point of view of increasing the "locking up" of larger particles, but how do the two factors compare in affecting the final performance ?

Having spent some of my sleepless hours trying to work that out in my head, it seems to me now that it would greatly ease the general discussion if the relative effects of each of the many variables being considered could be listed in order of importance to the final strength.
A rough "order of magnitude" figure would also help people decide at what stage they could start experimenting with a set up to achieve their own design.
What think thou ?

John

[ckelloug]

John,

Your last paragraph is the most profound question that has been asked on the thread in a long while. I skimmed through a dozen books on composite materials and concrete and some journal articles on aggregate today in Salmon Library at University of Alabama at Huntsville trying for an answer.

The impression I got from spending the day in the library was that academic interest in epoxy concrete died off in the early 1980's. Interestingly enough, it also seems that concrete engineers stopped doing research on aggregate designs in about 1907. The standard designs are embodied in a bunch of books from the American Concrete Institute. There's one french guy called de Larrard who has researched aggregate in modern times and his book is about $300 on amazon unavailable in any library anywhere near me. http://www.amazon.com/dp/0419235000?...GNWTRPVBYRP7T&

As best I can tell, there is a huge gap between the materials scientists who try to understand materials from first principles and the engineers who make the concrete that composes everyday items. The gap between understanding and doing is filled by a whole bunch of empirical test procedures which aim to determine whether a given material will work for a given application.

My posts here have been trying to apply materials science to the problem. As best I can tell, much of the theory that we have tried to apply hasn't been applied to epoxy concrete for 20 or 30 years and a lot of the arguments on this board are somewhere between rediscovering and discovering results that are not widely known.

Most of the material on Portland Cement concrete and aggregates I've seen is about trying to reduce the undesirable properties of the water/cement ratio and the Portland Cement behavior. They generally don't even care about the strength of their aggregates. Aggregates appear to be picked on the basis of availability in the PC concrete world and as long as they don't contain clay, anything will work.

The aggregate sizing decisions that the PC folks make seem largely related to controlling the amount of water added to the mixture to make it flow which destroys the properties of the matrix. It was said in one of the handbooks I looked at that most decisions made about PC concrete affect two material properties but the effect of the largest change is the only one noticed.

Because E/G is a different material that PC concrete, most of the existing PC aggregate designs aren't much better than a dice throw in E/G where the water problems of PC don't plague us. These aggregate designs also leave out the 100 years of materials science that has happened since 1907.

The things that I did find in vairous books mostly noted empirically without proof were the following:
<OL>
<LI>Sand (not necessarily rocks) greater than 5mm in diameter is usually of very low strength and should not be used. (Applicable to PC and E/G)
<LI>Fluvioglacial gravels that were smashed by a glacier tend to provide the strongest natural aggregate.
<LI>Synthetic aggregate produced by blasting and crushing is the strongest because the huge forces that have already been applied weed out pieces with critical flaws.
<LI>Fly ash contains mainly SiO2 Al2O3 and Fe2O3
<LI>Joost C. Walraven's 1981 Journal of the Structural Division paper says that the big failure mode in PC concrete is in the bond between the PC matrix and the aggregate. He also indicates that in PC concrete the crack will form in the matrix and propagate around aggregates.
<LI>Titanates, ZircoAluminates and Polysiloxanes are the most used bonding agents.
<LI>Silica Fume and Carbon Black are just beginning to be used as common reinforcing components in PC concrete.
<LI>Polysiloxane based bonding agents branch more when in the presence of aluminium oxide based admixtures.
<LI>Wollastanite is a good filler material
<LI>Chromium Oxide and Cobalt Silicate catalyze epoxy cures
<LI>Mica catalyzes epoxy curing
<LI>Carbon black is used as a filler in polyethylene at 50% fill ratios!!!!
<LI>High proportions of carbon black and silica fume adsorb chemical components in the epoxy mixture and make the cure much slower than it otherwise would be.
<LI>Carbon black is used to enhance electrical and mechanical properties in polymers and was specifically listed in the handbook as a suitable admixture for epoxy.
<LI>Aggregate of a form they call "cubic" where the pieces are of similar size in all directions are preferable and usually a lot stronger than pieces that are "rectangular".
<LI>Round rocks like river rocks were used in older concretes. It was because they were available, have excellent flow properties and lack of stress concentration points. New concrete generally uses manufactured aggregates which are much more angular because much of the natural gravel supply was exhausted. Some books also seemed to think rough aggregate sticks better.
<LI>Smaller aggregate is generally stronger than larger aggregate.
</OL>

On the topic of the close packed aggregate, I can't quite get the calculation to work out to anything at all. I'm too tired I think so I'll try again on getting a number for your question tomorrow.

There are a number of factors that affect the ultimate E/G product and I would like input on factors I overlook in the following list. We can sort the list later and see how it goes.

<B>Aggregate</B>
<UL>
<LI>Fracture Toughness of Aggregate Material Chosen
<LI>Aggregate Size Distribution Relative to Largest Aggregate in mixture
<LI>Size of Largest Aggregate in mixture
<LI>Solid Phase pH of Aggregate surface
</UL>

<B>Epoxy</B>
<UL>
<LI>Epoxide Equivalent Weight of Epoxy
<LI>Hardener family (amine etc.)
<LI>Reactive Dilutants
</UL

<B>Bonding Agents</B>
<UL>
<LI>Bonding Agent choice: Polysiloxane, Titanate, Zircoaluminate
</UL>
<B>Dispersion Hardeners</B>
<UL>
<LI>Dispersion hardener choice: Carbon Black, Silica fume, Epoxy Dispersed Colloidal Silica Sol
<LI>Dispersion hardener weight fraction
</UL>

<B>Catalysts</B>
<UL>
<LI>Choose catalyst: None, Cobalt Acetyl Acetonate, Mica dust, Chromium Oxide, Cobalt silicate
</UL>

<B>Mechanical Qualities of Mixture</B>
<UL>
<LI>Percentage and size of voids
<LI>Void location: Entrapped in matrix or surrounding aggregate
<LI>Voids smaller than Griffith flaw for part stress
<LI>Vaccum deairing
<LI>Effective compaction
<LI>No material seggregation
<LI>Epoxy setting temperature: higher is better
<LI>Avoidance of shear planes


<h4>Please post missing factors so we don't miss anything!!!!</H4>

[lgalla]

Aggregating is it not?100 years and the perfect concrete aggregate mix is not "cast in stone"Smooth river rock is probably best,but Cameron point's out lack of supply.Most concrete I have looked at has large voids.E/G is a difficult subject but I see progress in the posts to an answer.Solutions are forthcoming.
Larry

[greybeard]

Thanks Cameron for spending another(?) day in the library and producing lots to chew over.
Your last post rather underlines the number of variables, and consequently reinforces the need for some "gathering together" in a simplified form, so that each can take according to their needs.
I followed up your link to the de Larrard book, and got quite interested in reading the sample chapter that Amazon post up - pity it didn't go further !

I also found myself absorbed by the glacial deposit locations here in Norfolk something that I never imagined I would ever write( for your education that's the top half of the big round bump on the right side of the map of UK ).

I finished up reading your post with a strong impression that new ground is being turned over here. I can see how easy it would be to take on board the concrete tradition, and to assume that it would be valid to exchange epoxy for water and cement, and use the aggregate as before.
Very tempting, but I begin to realize what a false assumption that is.

A slight digression.
Hopefully more people will start to produce samples, so it would be a useful step for a simple testing rig to be designed, one based on common "junk box" contents.
It would allow everyone at that stage to compare their own progress with improvements in their own recipe, but also with others on opposite sides of the globe !
It would require a standard size to be set, or on second thoughts, perhaps two or three "standards" depending on the format/size of machine being aimed at.
I haven't thought through a complete design, but I would begin with steel rods as fulcrums, and a laser pointer with mirror on the sample, such that one gets some great optical leverage for measuring deflection.
Perhaps others of a like mind would add their thoughts and ideas.
Regards
John

[rockingjess]

Hi there!
I´m trying to make or pour a countertop from any epoxic or polyester resin to be able to stand some solvents,and the epoxy-portland mix,seems to be the answer.
Since Idon´t have much experience with epoxyes I need some help with ratios or quantities to mix a slab 1 inch thick by 4feet square ,that is ,1"x4´x4

[lgalla]

Cameron,Wollastanite appears to be a good filler as its aspect ratio provides re -inforcement but at the expence of hardness.4.5 on the Mohs scale.
Double that for mica flakes whose aspect ratio appears to re-inforce,but it has extreme softness.2.5 on the Mohs scale.
From your extensive research,in your opinion are spherical and smooth rounded aggregate best?I assume smooth aggregates will entrap less air and help de-gassing.Sharp angular aggregate will have an aspect ratio to provide re-inforcment,but concentrate stresses to the points,possibly causing micro cracking.Spheres tend to arrest micro craking.
Thanks for all the time you spend on the issues.
Larry

[ckelloug]

Rockingjess,

There are various bits of information in this thread about such things but I don't think there's a good answer here for you yet.

Given the problem that you mentioned, I would imagine that buying a piece of this material would be a lot easier and less costly than the amount of setup it will take to do an adequate job of making your own piece out of epoxy and aggregate:

http://www.fireslate.com/pages/home.cfm

If you have more specifics, post them and perhaps someone will be able to provide you some more information.

Regards,

Cameron

[ckelloug]

We will start with Aggregate:

<B>Aggregate</B>
<UL>
<LI>Fracture Toughness of Aggregate Material Chosen
<LI>Aggregate Size Distribution Relative to Largest Aggregate in mixture
<LI>Size of Largest Aggregate in mixture
<LI>Solid Phase pH of Aggregate surface
</UL>

I notice that there are many folks who are looking at the Mohs Scale of Hardness values as an aggregate selection criteria. This is really only a rough estimate of surface hardness and has no guarantee of correlating with performance of the material under these circumstances.

Materials Science in the branch that studies fracture in hard materials gives us a different criterion called the "fracture toughness". Higher fracture toughnesses equates to materials that are harder to break. The value of fracture toughness is more important as best I can discern than the round or angular nature of the aggregate. The units for fracture toughness are either psi sqrt(inch) or MPa sqrt(meter)

Here's a quick rundown from a couple of sources on fracture toughness in MPa sqrt(m) so that we can compare these aggregates effectively:

Synthetic Fused Quartz 0.7
Spinel 1.3
Chalcedony 1.3
Chert 1.4
Flint 1.4
Agate, Banded 1.8
Sapphire 2.1
Aluminum Oxide 3.9
Silicon Carbide 4.0
Silicon Nitride 4.0

The above fracture toughness values are from the work of Wood and Weidlich in the following paper:
http://www.minsocam.org/ammin/AM67/AM67_1065.pdf

From the NIST cermaics webbook

Titanium Diboride 5.2
Zirconium Oxide Yttrium Oxide 10

For interest's sake,

Metallic aluminum is about 30
Polycarbonate (Lexan) is about 3.6

I expect that the values for garnet and some other materials would be very good but I haven't felt like spending the money to subscribe to <U>The American Minerologist</U> to get the data although it would be cheaper than the gas to get to the nearest library with a copy. They put their older issues up for free but they want you to join the society and pay for the new stuff. The have one of the most reasonable polices of any of the societies and some of the lowest journal costs however taxes are due in a few days for me so not this week . . . There's a paper in there from 2007 by a Margaret Broz with values for garnet as well as some other nice papers lately.

It needs to be pointed out that these numbers tend to improve as particle sizes go down although once you're into the micrometer sized particles, it may be a moot point.

Exactly how strong that the aggregate needs to be is dictated by the bond strength between the aggregate and the epoxy. If the aggregate is much much stronger than the epoxy, it will never reach anywhere near it's critical stress and if it was expensive to get aggregate that tough, you don't get good value for the money spent. Stronger in everything is generally good but there is a definite limit. The only certainty is that of almost all of the possible candidates for mineral aggregates that we can get cheaply as abrasives, quartz is the worst!

<B>Epoxy</B>
<UL>
<LI>Epoxide Equivalent Weight of Epoxy
<LI>Hardener family (amine etc.)
<LI>Reactive Dilutants
<LI>No Non-reactive dilutants
<LI>Fracture Toughness value of the epoxy
</UL>

Ideally, as strong an epoxy as one can find is optimal provided it's cost-effective and that it's possible to mix the stuff and get the aggregates dispersed. What is chosen for epoxy will be highly dependent on the mixing techniques we have available and the aggregate sizes involved. The chosen epoxy should contain no non-reactive dilutants although reactive dilutants that aren't ridiculously toxic are fine. Ultimately,the fracture toughness of the epoxy governs the ultimate strength of the part if the aggregate does not fracture. If anybody has experience with epoxy chemistry, please expand upon or correct this.


<B>Bonding Agents</B>
<UL>
<LI>Bonding Agent choice: Polysiloxane, Titanate, Zircoaluminate
</UL>

Bonding agents are chemicals that have affinities for both the chemistry of the aggregate and the matrix. The handbooks I saw suggested these 3 although practically speaking, an epoxide group siloxane like dow z6040 seems ideal. In general it appears that epoxy makes it easy to choose bonding agents and aggregates as its chemistry is very compatible with more than most adhesives.

<B>Dispersion Hardeners</B>
<UL>
<LI>Dispersion hardener choice: Carbon Black, Silica fume, Epoxy Dispersed Colloidal Silica Sol
<LI>Dispersion hardener weight fraction
</UL>

Dispersion hardeners are a class of compounds used in advanced materials to pin dislocations in the crystal lattice. Dispersion hardeners are characterized by particle sizes below 1 um. The movement of dislocations tends to make a material flexible and ductile while a dispersion hardener tends to block these movements increasing the modulus and sometimes the strength. Dispersion hardeners are not very commonly used in concrete yet because the science behind them is relatively new. Silica Fume only became available after this microscopic nuisance dust was required by regulations to be collected for air pollution control. Eventually they figured out that it had some really helpful properties and it started gaining acceptance in concrete. Carbon black has been around for hundreds of years and is used as a dispersion hardeneing agent in rubber. It is used in epoxy as a colorant and to improve electrical properties and in theory should work well as a dispersion hardener although the large fractions that are necessary drastically increase the cure time due to adsorption of the hardener.


<B>Catalysts</B>
<UL>
<LI>Choose catalyst: None, Cobalt Acetyl Acetonate, Mica dust, Chromium Oxide, Cobalt silicate
</UL>

These substances increase the reaction rate in the epoxy hardening reaction. Cobalt Acetyl Acetonate is also known to improve the crosslinking which results in higher strength and which also raises the glass transition temperature of the epoxy and thus the temperature at which the cured item may be used.


<B>Mechanical Qualities of Mixture</B>
<UL>
<LI>Percentage and size of voids
<LI>Void location: Entrapped in matrix or surrounding aggregate
<LI>Voids smaller than Griffith flaw for part stress
<LI>Vaccum deairing
<LI>Effective compaction
<LI>No material seggregation
<LI>Epoxy setting temperature: higher is better
<LI>Avoidance of shear planes

</UL>
The biggest problem with the epoxy is voids. They are worse than with Portland Cement due to the fact that small aggregate is more convenient and stronger and epoxy is more viscous and less wetting than water. Brittle materials fail via crack propagation governed by the Griffith equations.

A simple void in the epoxy, which I will call a type 1 void is a bubble of air in the epoxy matrix that it adjacent to aggregate. From the griffith equations, a type one void will cause no appreciable problems as long as its size is smaller than that prescribed by the equations for the stress level that the void experiences in the completed part. If this type one void is larger than the critical flaw size for the stress level then the part is likely to fail suddenly and catastrophically when that stress is applied.

I hypothesize without having seen this in the literature that there exists another type of void. I call this a type 2 void. A type 2 void occurs when air that is adsorbed or otherwise held by an aggregate particle forms a shell around the particle. In this case, the particle has little or no mechanical connection to the matrix. A type 2 void both provides a griffith flaw for some stress level in the epoxy and it also causes that particle to provide no reinforcement. It is my belief that the presence of type 2 voids explains the failures that are associated with batches using the smallest aggregates. If an epoxy and micrometer aggregate mixture were produced without adequate deairing, the net result would be a bar of unreinforced epoxy filled with griffith voids and waiting for failure. Since the air bubbles around adjacent particles could coalesce, the effect might be to create griffith voids that would propogate to catastrophic failures at extraordinarily low stress levels.

It is my current belief that the liklihood of type 2 voids goes down as the aggregate gets bigger but the severity of the consequences of one occuring goes up. A piece riddled with type 2 voids will be useless.

The shear plane effect is caused when the aggregate is not distributed properly in the matrix. If the aggreate is not distributed randomly then it is possible for a crack to form which is not impeded by aggregate particles. Such a crack is blocked by the fracture toughness of the epoxy matrix alone and will likely propagate to failure. Other than the relatively unattainable nature of a close pack solution, preventing distinct shear planes is a prime reason for widely distributed aggregate sizes.


<h4>Please post missing factors so we don't miss anything!!!!</H4>[/QUOTE]

[gerryv]

I've not followed the whole thread, it's long, but if it hasn't already been mentioned you might want to check out a product called "Ductal", a super concrete. - Gerry

[ckelloug]

Gerryv,

The ductal product you sent the link on is interesting and has properties similar to the E/G product we are researching making and in some cases better properties. It is a commercial product and I would assume it is expensive. I didn't get a sense of the vibration damping qualities from my perusal of the spec sheet. This is however one of the first engineered concrete products I've seen that starts to incorporate materials science instead of brute force pour and test techniques.

We're trying to make an inexpensive epoxy composite out of cheaply available materials so this isn't what we're doing but it's a good read none-the-less.

Thanks for the post.

[PlasticWorker]

As you may have noticed from ckellong's link to Firestone, most if not all concrete counter tops are made from a fairly standard concrete / water mixture. You can make your own if you are so inclined.
Make a mold from plywood pre laminated with melamine or "formica". Two inches is a good thickness.
Use dry colors to make a shade you like.
Use as little water as possible. Acrylic bonding agent also helps the strength.
Tamp and screed the the mixture into the mold.
After it cures for a week, demold and turn right side up. Scrub the surface. after it drys, seal with a concrete sealer. ( It will get darker).

Sorry to all of the on topic posters, just trying to help.

[walter]

Hi everyone, just a quick update on the name change.

I wanted to start a separate "Epoxy-Granite" thread to better reflect what we're doing here, but was advised against it. Then the name change idea came up and the Moderators sort of went with it (and I appreciate that very much). They also helped us with an Index to this particular thread and made it into a Sticky. The Index will help you navigate through the main thread, find documents, pictures, etc. Please continue to post here- the other thread is for indexing purposes only.

On or off topic- it's all welcomed!
We want this to be the friendliest place on the Zone

Thanks for all the suggestions and feedback.

Cheers!
_

[greybeard]

Gerryv,

The ductal product you sent the link on is interesting and has properties similar to the E/G product we are researching making and in some cases better properties. It is a commercial product and I would assume it is expensive. I didn't get a sense of the vibration damping qualities from my perusal of the spec sheet. This is however one of the first engineered concrete products I've seen that starts to incorporate materials science instead of brute force pour and test techniques.

We're trying to make an inexpensive epoxy composite out of cheaply available materials so this isn't what we're doing but it's a good read none-the-less.

Thanks for the post.

I've just read one of their technical library articles published in 2002 which gives an interesting description of the way they've developed an increase in the "toughness" and "strength" of high performance concretes, (I thought they meant the same till I read the article !), by working on all the components.
Although the product is very different from what is being attempted here, the difference between the traditional idea of a concrete mix and what they have developed is quite remarkable - out go large aggregates, in come fibers of various types, for example, - and while it's dangerous to argue by analogy, it does suggest that there might be lessons to be learned, and radical new ideas to be discussed and tried.
And the emphasis has to be on "tried"......

John

[ckelloug]

Greybeard,

Fibers are another very valid method of reinforcement. I saw that the main ductal product was reinforced with metal fibers. The governing equation for fibers is different and the required chemistry to get good adhesion might be different but they are very much composites. The aerospace industry has a lot of fiber composite experience and there is a lot of research in epoxy-fiber composites.

The 1 second intro to fibers is that they must be longer than a critical length which is about an inch for carbon and glass fibers to be used to maximum effect. I don't know the critical lengths for metals off the top of my head. Materials can be made with either short or long fibers and a piece made with short fibers that are parallel but not a single piece can be 95% as strong as a piece that is made with solid fibers in it. For randomly orinted fibers, the piece is 1/5th as strong as a piece made with full length fibers.

The principles behind epoxy carbon fiber and epoxy glass fiber are well documented in books in any university library. I don't really know how they compare in vibration damping performance but I do suspect the cost is higher.

Regards,
Cameron

[ckelloug]

Materials have a number of properties which have similar sounding names and have a goal of confusing people. The following all are distinctive material properties.

<OL>
<LI>Tensile Strength
<LI>Tensile Modulus
<LI>Compressive Strength
<LI>Compressive Modulus
<LI>Flexural Strength
<LI>Flexural Modulus
<LI>Poisson's Ratio
<LI>Fracture Toughness
</OL>

Tensile strength is the property of a rope when you pull on both ends until it breaks.

Tensile Modulus is the ratio of the force applied per square inch to the elongation.

Compressive strength is the property of a piece of new bubblegum which represents the force it takes to see the surface crack.

Compressive modulus is the ratio of the force applied per square inch to the shrinkage in the direction of the applied force in the chunk of bubblegum.

Flexural Strength is the strength required to break a pencil in half when it is supported at the point and the eraser ends and a force is applied in the middle.

Flexural Modulus is the ratio of the force applied per square inch of cross section of the pencil to the amount that the center moves towards the desk when the force is applied.

Poisson's ratio describes the amount that the chunk of bubblegum gets wider in one direction when it is squashed in another.

Finally, fracture toughness is the material property that describes how much mechanical work it would require to cause a crack in a brittle material like glass to continue all of the way through the material and severe the piece. (Almost all the materials that we deal with on this thread are brittle except for metals).

<B>Putting the Pieces together</B>

In E/G, the fracture toughness of both the aggregate and the epoxy define what the flexural strength of the material will be.

Tensile strength is of minimal interest as most machine designs don't use tensile loading. Compressive strength is good to know but usually requires forces too high to be testable without large presses. Flexural strength, the only one that is both relatively useful and measurable is the "strength" that governs most E/G machine parts like beams and tables.

<B>Stretching: the Truth</B>

The biggest problem in making E/G metal cutting machine parts is that a beam of E/G with weight on it will sag in the middle. (All materials will sag but E/G sags more). This sagging is proportional to the flexural modulus. It is very possible to make a beam that is quite strong but sags a bit under load (not good for a machine tool but okay for a house). It is also very possible to make a beam that is much more rigid than the first beam but fails when only a few pounds of load are applied (also bad since that might free a 10hp router to fly around the shop). Flexural modulus of an E/G part is most affected by the tensile, compressive, and flexural moduli of the aggregate and the same parameters in the epoxy.

<B>The School of Hard Knocks</B>

To make epoxy granite that is difficult to break, the fracture toughness of the aggregate and the epoxy are the most important. Synthetic Fused Quartz is inferior to natural quartz which is inferior to flint which is much inferior to aluminum oxide. Brittle materials fail by cracking through rather than by stretching like chewed bubble gum where the strand becomes thinner and eventually breaks. A general trend amongst the small particle aggregate under consideration is that materials with a high fracture toughness also have a relatively high modulus.

The most important thing to remember is that smaller aggregate generally has a higher fracture toughness than larger aggregate. The general reason is this: any piece of aggregate of a given size most likely broke off of a larger piece of aggregate when a critical flaw in the larger piece reached it's griffith stress. As a result, each time an aggregate breaks under an applied load, most of the flaws for a given stress level are eliminated by the fracture and all remaining flaws in the fractured pieces have a higher griffith stress than the original piece. If this is repeated until the aggregate pieces become very small then the tiny pieces of aggregate end up being very difficult to further break.

<B>Keeping it Together</B>

All of this talk about measurements and aggregate means that it is necessary too to consider the interface between the matrix and aggregate. This is most often ignored by materials scientists because the interaction itself is mainly a chemical affair involving covalent and hydrogen bonds between the matrix, the bonding agent and the aggregate.

In general, a bonding agent such as a silane based agent works by having a chemical group with mineral properties on one end of the molecule and adhesive properties on the other end. Dow Z6040 has silica on one end and epoxy on the other for example.

Epoxy tends to be very versatile and will bond with most surfaces that have either acidic or basic chemical groups attached. The main place where this doesn't work is in long chain polymers whose regular structures do not contain chemical groups that interest the epoxy molecules.

That being said, most epoxy will stick to most aggregates as long as everything is free of contaminants. It should also be noted that most epoxy sticks to most aggregates a lot better when it has a silane with a high chemical affinity for the aggregate providing a bonding site.

<B>Putting it all together</B>

An epoxy with as high a tensile strength and as low a viscosity as practicable when mixed with an aggregate with as high a fracture toughness as practicable, combined with a bonding agent that has an epoxy ring on one end and a siloxane group on the other is likely to perform maximally well when accompanied by a dispersion hardener.

<B>Remaining hurdles</B>

The remaining hurdles in order of severity are:

<OL>
<LI>Aggregate Size Distribution
<LI>Aggregate Maxium Size
<LI>Thorough Deairing of the mixture to avoid type 2 voids
</OL>

We can tell from fracture toughness that smaller aggregate is stronger. We know from Gupta and Gamski that there is an ideal average epoxy thickness over the estimated aggregate surface areas around 30um. This leads to the possibly unwarranted assumption that particles near in size or smaller than 30 micrometers make the total epoxy required go up quickly and we know from the rule of mixtures calculation that increasing the epoxy fraction beyond this point is bad for all parameters.

Barring rheological considerations about the material being pourable or transferable from its container, it seems that the practical limit is probably about 60um at the low end except for dispersion hardeners and 5mm at the high end based on the handbook comment that 5mm sand generally has awful fracture toughness.

Because it is generally impossible to achieve the dense pack figure of 75% for a single size of aggregate, it is necessary to use multiple sizes. The handbooks I saw indicated that Fuller's ratio is tuned for some of the water and aggregate problems in PC concrete rather than maximum possible strength and modulus. AS a result, it appears likely (at least to me) that the best distribution will either come from further study of the NISS paper on statistical aggregate simulation or research from the rather expensive and rare book on the subject by de Larrard posted earlier.

Deairing is a simple matter of using vaccum and possibly deairing agents and people can draw their own conclusions on what works for them with the concensus seeming to be that less than 28 inches is probably ineffective and more than 29.75 inches is overkill.

Regards to all and especially John for asking the question that promulgated this post,

Cameron

[lgalla]

:violin: I feel badly for quartz being outlawed as a good aggregate.Good or high quality granite is 60 or 70 % quartz.Does this also outlaw granite as a filler.Copy from a paper"
Synthetic Fused Quartz 0.7
Spinel 1.3
Chalcedony 1.3
Chert 1.4
Flint 1.4
Agate, Banded 1.8
Sapphire 2.1
Aluminum Oxide 3.9
Silicon Carbide 4.0
Silicon Nitride 4.0"
Synthetic fused quartz is basically melted quartz,or glass.Most items on the chart are gemstone minerals.You cannot walk in to a supplier and get a bag of spinel.Surfing for fracture toughness on granite produces little info.
Cast iron is full of micro cracks and the reason for damping properties.
Some machine builders use quartz as a high end aggregate.Nearly all sands and small aggregates will have high quartz content.In our case,is fracture toughness the ability to resist hammer blows or forklift bumbs.
Fracture toughness of aluinium is 36Mpa,steel50Mpa.From the Mpa's they appear to be better choices,but we know their damping sucks.Steel or alu will bend of flex to reduce fracture cracking as they are ductile.Ducile= vibration.We are attempting to reduce vibrations with extremely stiff materials.Hope someone understands my jist,I don't
Larry

[ckelloug]

Larry,

I wouldn't outlaw quartz or granite just yet. They are just more likely to crack under stress. Very similar to engineers My comments on strategy above are based on making an epoxy aggregate combination with the absolute maximal strength. Maximal strength is likely not be necessary for most E/G machine parts but I find it interesting to explore that part of the problem space because making the minimum strength material appears to have already been accomplished.

The precise characteristics for transmitting vibration are relatively complicated. From my understanding, it is a combination of the density, the damping properties of the material, and the modulus that are the main factors. As metals go, Aluminum is one of the worst. I am not sold on ductility being the prime cause of vibration transmission.

Because epoxy has a low modulus and a much higher viscous damping than metals or aggregates, even epoxy diamond would probably have good damping. Epoxy diamond would probably not do as well from a fracture toughness standpoint as say Epoxy Boron Nitride however But before I digress and get more silly. . .

I cited the mineral data for fracture toughness because it was available and depending on the exact location of the quartz quarry you are using, it may behave similar to the other materials or be sold as quartz. The paper from the minerologists called them forms of quartz.

Many of the others like garnet (for which I do not have data) as well as silicon carbide and silicon nitride are available as abrasives and thus probably cost effective on the order of quartz.

I've blithered enough on this thread lately and it's once again time to quit for the day.

--Cameron

[greybeard]

Time for a party - we've just passed the 1500 post mark, so to entertain you briefly, a link to something very silly but somehow wonderful.

http://www.youtube.com/watch?v=D2f1KEEtRyk

John